Whole blood hematocrit is a measurement of the volume fraction of the corpuscular components in a blood sample. Hematocrit can be estimated from a measurement of the conductivity of a sample of blood, since blood plasm is a relatively good conductor of electricity, while the blood cells are relative insulators. Various instruments have been developed which incorporate conductivity cells through which a blood sample is flowed to measure the conductivity.
A type of conductivity measurement, known as a four terminal or Kelvin connection, comprises a pair of current carrying electrodes spaced on either side of a pair of voltage measuring electrodes. This technique, in which the voltage measuring electrodes are separated from the current carrying electrodes, enables only a low current to be drawn from the sample, thereby having a negligible effect on the path of the constant current flowing between the current carrying electrodes.